Process for preparing concentrated aqueous micellar solutions

ABSTRACT

The present invention provides a method for preparing supersaturated aqueous solutions of micelles of compounds in which the solubility in water is less than the critical micelle concentration. The solutions can be process to prepare solid micelles which have advantageous properties for drug delivery.

CROSS REFERENCE TO PRIOR APPLICATIONS

This application claims the benefit of priority to U.S. Ser. No.61/033,529 filed Mar. 4, 2008 which is hereby incorporated in itsentirety by reference.

FIELD OF THE INVENTION

The present invention relates to a process to prepare supersaturatedmicellar solutions of biologically active molecules with low waterinsoluble that are useful to prepare novel formulations.

BACKGROUND ON THE INVENTION

Drug discovery programs frequently identify molecules with highbiological activity and suboptimal physical properties that result inlow bioavailability. Modification of physical chemical properties,mainly solubility and dissolution rate, may alter the pharmacodynamicand pharmokinetic properties of a compound. Traditionally modificationof properties such as solubility, dissolution rate, hygroscopicity,stability and crystal habit was approached by forming salts of ionizablemolecules with a variety of pharmaceutically acceptably counterions.More recently polymorphs and pseudopolymorphs have been screened toidentify crystalline forms with improved physical chemical properties.Typically the crystal structure of different salts and polymorphs, andtherefore the physical properties, differ. Co-crystals afford yetanother technique to identify new crystalline materials.

While traditionally crystalline salts were sought, more recently,amorphous forms of active pharmaceutical ingredients have beeninvestigated. Unlike crystalline solids which are comprised of regulargeometric patterns or lattices, amorphous solids are comprised ofrandomly oriented molecules. Common examples of amorphous solids areglass and plastic. Unlike crystalline solids, amorphous solids do nothave definite melting points and have a faster dissolution rate andgreater solubility than crystalline forms. One difficulty in usingamorphorus solids in formulations is there tendency to revert to a morestable crystalline form.

Other techniques for improving solubility and dissolution rates includemodifying crystal properties by micronization and nanosizing of thecrystals.

SUMMARY OF THE INVENTION

The present invention provides for a process for preparing asupersaturated aqueous solution of micelles from an amphiphilic compoundwhose solubility product (K_(sp)) in water is less than the criticalmicelle concentration (CMC) in water which process comprises the stepsof:

-   -   (a) dissolving an amphiphilic compound in a water miscible        organic solvent;    -   (b) adding water, and optionally a stoichiometric quantity of        aqueous alkaline or alkali metal hydroxide or aqueous acid, to        form a salt, to provide a homogenous mixed aqueous solvent        system;    -   (c) heating the solution under reduced pressure at a temperature        which results in distillation of the organic solvent to produce        a supersaturated aqueous solution of micelles and less than 0.5%        of the organic solvent.

The resulting supersaturated solution of micelles can be furtherprocessed by conventional techniques such as lyophilization or freezedrying to afford a solid which can be incorporated into conventionaldosage forms.

Molecules with aqueous solubility less than the critical micelleconcentration (CMC) are difficult to aggregate into micelles. Thepresent invention affords a process to produce concentrated micellarsolutions when the solubility is less than the CMC.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a photomicrograph of a spray dried (FIG. 1 a) and lyophilized(FIG. 1 b) concentrated micellar solution of the disodium salt of Iprepared as described in example 1 demonstrating different morphologiesfor the solid obtained from both drying techniques.

FIG. 2 is an x-ray powder pattern of the solid obtained by spray dryingthe concentrated micellar solution of the disodium salt of I prepared asdescribed in example 1 which establishes the compound does not have aregular crystalline structure.

FIG. 3 is a photomicrograph of a spray dried concentrated micellarsolution of compound II prepared as described in example 4.

FIG. 4 is an x-ray powder pattern of the solid obtained by spray dryingthe concentrated micellar solution of compound II prepared as describedin example 4.

DETAILED DESCRIPTION OF THE INVENTION

The phrase “a” or “an” entity as used herein refers to one or more ofthat entity; for example, a compound refers to one or more compounds orat least one compound. As such, the terms “a” (or “an”), “one or more”,and “at least one” can be used interchangeably herein.

As used in this specification, whether in a transitional phrase or inthe body of the claim, the terms “comprise(s)” and “comprising” are tobe interpreted as having an open-ended meaning. That is, the terms areto be interpreted synonymously with the phrases “having at least” or“including at least”. When used in the context of a process, the term“comprising” means that the process includes at least the recited steps,but may include additional steps. When used in the context of a compoundor composition, the term “comprising” means that the compound orcomposition includes at least the recited features or components, butmay also include additional features or components.

The term “optional” or “optionally” as used herein means that asubsequently described event or circumstance may, but need not, occur,and that the description includes instances where the event orcircumstance occurs and instances in which it does not. For example,“optionally substituted” means that the optionally substituted moietymay incorporate a hydrogen or a substituent.

The term “about” is used herein to mean approximately, in the region of,roughly, or around. When the term “about” is used in conjunction with anumerical range, it modifies that range by extending the boundariesabove and below the numerical values set forth. In general, the term“about” is used herein to modify a numerical value above and below thestated value by a variance of 20%.

While the crystalline and amorphous states represent extremes inmolecular order in the solid state, there can be a continuum ofpartially ordered liquid crystalline states that lie between theseextremes. (C. L. Stevenson et al. J. Pharm Sci. 2005 94(9):1861-80)These are solids with intermediate states of molecular ordercharacterized by a partial or complete loss of positional order whileretaining some degree of orientational order of the constituentmolecules.

A compound which has hydrophilic and hydrophobic regions within the samemolecule is amphiphilic. Soaps and detergents are common examples ofamphiphilic molecules. Amphiphilic molecules self assemble to formmicelles when their concentration in solution exceeds their criticalmicelle concentration. Many amphiphilic molecules show lyotropicliquid-crystalline phase sequences depending on the volume balancesbetween the hydrophilic part and hydrophobic part. A liquid crystallinematerial is lyotropic if the phases have long-range orientational orderThese structures are formed through the micro-phase segregation of twoincompatible components on a nanometer scale. Soap is an everydayexample of a lyotropic liquid crystal.

The content of water or other solvent molecules changes theself-assembled structures. At very low amphiphile concentration, themolecules will be dispersed randomly without any ordering. At slightlyhigher (but still low) concentration, amphiphilic molecules willspontaneously assemble into spherical micelles or vesicles. Micellaraggregates assemble to orient the hydrophilic portion of the amphiphileinside the core micelle, exposing a hydrophilic (water-soluble) surfaceto aqueous solution. These spherical objects do not order themselves insolution, however. At higher concentration, the still more orderedassemblies will form. A typical phase is a hexagonal columnar phase,where the amphiphiles form long cylinders (again with a hydrophilicsurface) that arrange themselves into a roughly hexagonal lattice. Thisis called the middle soap phase. At still higher concentration, alamellar phase (neat soap phase) may form, wherein extended sheets ofamphiphiles are separated by thin layers of water. For some systems, acubic (also called viscous isotropic) phase may exist between thehexagonal and lamellar phases, wherein spheres are formed that create adense cubic lattice. These spheres may also be connected to one another,forming a bicontinuous cubic phase.

Removal of the solvent from concentrated solutions of micelles producessolids which, depending on the drying technique and drying conditionsused, result in a liquid crystalline micellar solid or an amorphoussolid. Both amorphous and lyotropic liquid crystals are sufficientlydisordered that they do not produce sharp diffraction peaks in an X-raypowder diffraction pattern. These forms typical result in a halopattern. In contrast to amorphous solids, liquid crystalline phasesexhibit birefringence when viewed with a polarized light microscope.

Micellar solids afford some unique properties that can be exploited inthe development of novel formulations. Specifically the molecular orderprovided in liquid crystals, unlike an amorphous solid, adds stabilitythat can retard reversion to still more stable crystalline solids.However stability resulting from the aggregation is still significantlyless than the crystalline state resulting in increased solubility anddissolution rate typical of purely amorphous materials.

While micellar solids promise advantages to the pharmaceuticalscientist, the physical properties of many pharmacologically activemolecules make to difficult produce concentrated micellar solutions fromwhich liquid crystalline solids can be recovered. One reason for thisdifficulty is the aqueous solubility of many molecules is sufficientlylow that the CMC cannot be attained and thus micelles do not form. Thepresent invention provides a convenient process to produce concentratedaqueous solutions of micelles from compounds with limited aqueoussolubility.

The term “amphiphile” as used herein refers to a chemical compoundpossessing both hydrophilic and hydrophobic properties. Such a compoundis also referred to as amphiphilic or amphipathic. The hydrophilicportion of an amphiphilic molecule can be cationic, anionic or neutral.Neutral hydrophilic residues are commonly polyethers are similarresidues capable of hydrogen bonding. The hydrophobic portion of anamphiphile is typically comprised of alkyl or aryl residues

The term “micelle” as used herein refers to an aggregate of amphiphilicmolecules dispersed in a liquid. A typical normal phase (oil-in-water)micelle in aqueous solution forms an aggregate with the hydrophilic“head” regions on the exterior surface in contact with surroundingaqueous phase and hydrophobic tail regions sequestered in the center ofthe micelle where the environment is relatively non-aqueous. Micelles indiluted solutions are approximately spherical in shape. More complexliquid crystalline phases can be formed as micellar solutions becomemore concentrated and the shape and size of such micelles is a functionof the molecular geometry of its surfactant molecules and solutionconditions such as surfactant concentration, temperature, pH and ionicstrength.

The term critical micelle concentration (CMC) as used herein is definedas the concentration of molecule above which micelles are spontaneouslyformed. The term “mixed aqueous solvent system” as used herein refers asolution of water and a miscible organic solvent which which can beselectively distilled from the mixed solvent system. Typical organicsolvents include ethers, such as tetrahydrofuran (THF), dioxane or1,2-dimethoxyethane (DME), or alcohols, such as methanol (MeOH), ethanol(EtOH) or isopropanol (IPA), or acetonitrile. Typically the organicsolvent is chosen to provide a homogenous aqueous solution containingthe biologically active compound.

The term “liquid crystal” as used herein refers to a phase of matterthat has properties intermediate between those of a amorphous solid andthose of a solid crystal. A liquid crystal typically is comprised ofmolecules with some order but lacking the regular repeating subunits 6typical of a crystal lattice. When a liquid crystal is positionedbetween a pair of crossed polarizing filters and viewed through anoptical microscope a liquid crystal appears birefringent, i.e. thesample appears bright against a dark (isotropic) background.

In one embodiment of the present invention here is provided a processfor preparing a supersaturated aqueous solution of micelles from anamphiphilic compound whose solubility product (K_(sp)) in water is lessthan the critical micelle concentration (CMC) in water which processcomprises the steps of: (a) dissolving an amphiphilic compound in awater miscible organic solvent; (b) adding water, and optionally astoichiometric quantity of aqueous alkaline or alkali metal hydroxide oraqueous acid to form a salt, to provide a homogenous mixed aqueoussolvent system; and (c) heating the solution under reduced pressure at atemperature which results in distillation of the organic solvent toproduce a supersaturated aqueous solution of micelles and less than 0.5%of the organic solvent. One skilled in the art will appreciate that thequantities of water and the nature an quantity of the non-aqueoussolvent can be varied to provide mixed aqueous solvent systems whichdissolve the amphiphile and these quantities can be determined withoutundue experimentation.

The active pharmaceutical ingredient (API) is first dissolved in anorganic solvent and a quantity of water is added to produce anhomogeneous aqueous organic solution. An active pharmaceuticalingredient (API) with an acidic substituent can optionally be treatedwith a stoichiometric quantity of aqueous base to produce the conjugatebase of the acid and the resulting anion may enhance the hydrophilicityof the API. Analogously an API with a basic residue can be treated witha stoichiometric quantity of an aqueous acid to produce the conjugateacid which can enhance the hydrophilicity of the API. Conversion of theAPI to a salt is an optional component of the invention and is notrequired if the neutral API is sufficiently amphiphilic to formmicelles. The anhydrous acids or bases can also be a used to generatethe salt and water added in a subsequent step. The quantities of organicsolvent and water are typically adjusted produce a homogeneous solution.The organic solvent selected to be miscible with water and the boilingpoint of the organic should be low enough that the heat applied duringthe distillation not cause decomposition of the API.

The organic solvent is distilled under a vacuum to produce asupersaturated solution of micellar API. Distillation is continued untilthe solvent contains less than 0.5% of the organic solvent. Theconcentration of the API in water can readily adjusted. In anotherembodiment the distillation is continued to produce a supersaturatedsolution containing less than 1% of the organic solvent. In stillanother embodiment the distillation is continued to produce asupersaturated solution containing less than 2% of the organic solvent.

In a second embodiment of the present invention there is provided aprocess to prepare a stable amorphous solid micelle comprising spraydrying said supersaturated aqueous solution of micelles to produce astable solid amorphous micelle which exhibits birefringence under apolarized light microscope.

In a third embodiment of the present invention there is provided aprocess to prepare a stable amorphous solid comprising lyophilization ofsaid supersaturated aqueous solution of micelles to produce a stableamorphous solid.

The term “stable” as used herein refers to a physical form that isstable for at least about four weeks.

In a fourth embodiment of the present invention there is provided aprocess to prepare a stable amorphous solid micelle of a compoundaccording to formula I (S. Hirono, et al., WO 2003042150, published May22, 2003) comprising the steps of: (a) dissolving a compound accordingto formula I in THF; (b) adding two equivalents of 1M NaOH; and (c)heating the solution under reduced pressure at a temperature whichresults in distillation of the organic solvent to produce asupersaturated aqueous solution of micelles and less than 0.5% of theorganic solvent.

In a fifth embodiment of the present invention there is provided aprocess to a stable amorphous solid micelle containing a compoundaccording to formula I comprising spray drying the supersaturatedaqueous solution of micelles produced in the fourth embodiment (supra).

In a sixth embodiment of the present invention there is provided aprocess to prepare a stable amorphous solid of a compound according toformula H comprising the steps of: (a) dissolving a compound accordingto formula H in iso-propanol; and (c) heating the solution under reducedpressure at a temperature which results in distillation of the organicsolvent to produce a supersaturated aqueous solution of micelles andless than 0.5% of the organic solvent.

In a seventh embodiment of the present invention there is provided aprocess to a stable amorphous solid containing a compound according toformula H comprising spray drying the supersaturated aqueous solution ofmicelles produced in the sixth embodiment (supra).

The following examples illustrate the preparation and biologicalevaluation of compounds within the scope of the invention. Theseexamples and preparations which follow are provided to enable thoseskilled in the art to more clearly understand and to practice thepresent invention. They should not be considered as limiting the scopeof the invention, but merely as being illustrative and representativethereof.

Example 1 Preparation of an 3545% Aqueous Micellar Solution of DisodiumSalt of Compound I

A 12 L round bottom flask fitted with a mechanical stirrer andmaintained under a N₂ atmosphere was charged with I (601.5 g, 1.163 mol)and THF (ca. 4.8 L). The suspension was agitated at moderate speed andheated to approximately 60° C. to produce a homogeneous solution. Thesolution was cooled to room temperature (RT) and a 1M NaOH solution(2320 mL of a solution of 120 g NaOH diluted to 3 L with sterile waterfor irrigation which assayed as 0.985 M by titrametric analysis) wasadded with stirring. The temperature of the solution rose from 22° C. to30.8° C. This solution was vacuum transferred to a Buchi Rotovap (12Lflask). The THF was removed in vacuo (25-27 Torr). The water bathtemperature was maintained at 50-55° C., the vapor temperature was ca.21° C. and the flask was rotated at 67-72 revolutions per minute. As thevolume decreased, the temperature of the vapor increased to 35-36° C.The temperature of the water bath was raised to 60° C. to maintain thedistillation rate. When the vapor temperature had reached about 35° C.,the solution became cloudy and the solution was aged at about 60° C. for1 h to re-clarify the solution while slowly bleeding N₂ into the rotaryevaporator to prevent foaming and “bumping. When the temperature of thevapor remained constant at 36° C. a sample was removed and the pHdetermined to be 8.5. The solution was diluted with sterile water (ca.500 mL) and concentration of the solution was continued until the finalvolume reached ca. 1.5 L. Distillation was discontinued and the residualTHF concentration was determined to be 0.015%, the concentration of thedisodium salt of I was determined to be 44% (wt/vol) and the pH was ca.

8.5. The resulting micellar solution (1.726 kg) was stored in sterilebottles.

Example 2 Spray Drying of the Micellar Solution from Example 1

The pH of the solution from example 1 was adjusted to pH 9 with 1N NaOHand transferred to a Buchi B-290 spray drier and the operatingparameters were adjusted as follows:

inlet temperature—175° C. (resulting outlet temperature ca. 106° C.);

air pressure of the spray nozzle—ca. 30 psi;

spray pump speed—18-20%;

aspirator—ca. 80%.

Spray drying was begun when the inlet temperature reaches to 175 C.Typically the exhaust pressure is about 15 psi at 80% aspirator speed.After the solution was run through the spray drier the inlet heater pumpand air inlet are turned off and the aspirator reduced to ca. 50%). Whenthe exhaust temperature drops to 60-70° C. the aspirator is turned offand the powder is collected from the chambers. The resulting powder canbe dried in an oven to reach the desired moisture content.

Example 3 Lyophilization of the Micellar Solution from Example 1

A round-bottom flask was charged with a solution from example 1 andimmersed and swirled in a dry ice/isopropanol slurry to freeze thesolution. The flask containing the frozen solution was attached to alyophilizer to remove the water. Complete drying required 8-20 h. Thevacuum is broken and the flask removed and the resulting powdercollected. Any lumps can be broken with light pressure with a spatula orin a mortar and pestle.

Example 4 Preparation of an Aqueous Micellar Solution of Compound H

Compound II (2 g) was slowly dispersed in 70% IPA (20 mL) and water (14mL) was added. (The solubility of the II is about 47 mg/mL in 70%IPA/H₂O, 4 mg/mL in water and 0.08 mg/mL in IPA.) The dispersion wassonicated to dissolve all the dispersed solid to produce a pale yellowsolution. The IPA was distilled using rotary evaporator to achieve anaqueous solution containing ca. 400 mg/nL of II. The concentratedaqueous solution was spray dried using a Buchi B-290 spray drier and theoperating parameters were adjusted as follows:

inlet temperature −180° C. (resulting outlet temperature ca. 100° C.);

air pressure of the spray nozzle—ca. 30 psi;

spray pump speed—12%;

aspirator—ca. 90%.

Example 5 Stability Study of Micellar Solid from Spray Drying of I

A small amount of the solid (about 10 mg from example 1) was weighedinto a weighing bottle and placed in a chamber with controlled relativehumidity for 4 weeks and the percentage of water absorbed was calculatedform the weight gain. The sample was also assayed by HPLC against anexternal standard on a Waters 2690 HPLC at 276 nm. The data wasprocessed using Waters Millennium software version 3.2. The thermalstability also was determined at 60° C. and at 40° C./75% relativehumidity. The purity of the sample was determined by assaying weighedaliquots by HPLC against an external standard. The experiments suggestthat the micelles are not hygroscopic and are thermally stable at 40 and60° C. over the duration of the assay.

Area % CONDITIONS T_(o) 4 weeks 5 weeks 40° C./75% RH (open) 99.81 99.2640° C./75% RH (closed) 99.88 99.91 25° C./60% RH (closed) 99.88 99.8860° C. (closed) 99.81 99.72

The foregoing invention has been described in some detail by way ofillustration and example, for purposes of clarity and understanding. Itwill be obvious to one of skill in the art that changes andmodifications may be practiced within the scope of the appended claims.Therefore, it is to be understood that the above description is intendedto be illustrative and not restrictive. The scope of the inventionshould, therefore, be determined not with reference to the abovedescription, but should instead be determined with reference to thefollowing appended claims, along with the full scope of equivalents towhich such claims are entitled.

All patents, patent applications and publications cited in thisapplication are hereby incorporated by reference in their entirety forall purposes to the same extent as if each individual patent, patentapplication or publication were so individually denoted.

1. A process for preparing a supersaturated aqueous solution of micellesfrom an amphiphilic compound whose solubility product (K_(sp)) in wateris less than the critical micelle concentration (CMC) in water whichprocess comprises the steps of: (a) dissolving an amphiphilic compoundin a water miscible organic solvent; (b) adding water, and optionally astoichiometric quantity of aqueous alkaline or alkali metal hydroxide oraqueous acid to form a salt, to provide a homogenous mixed aqueoussolvent system; and, (c) heating the solution under reduced pressure ata temperature which results in distillation of the organic solvent toproduce a supersaturated aqueous solution of micelles containing lessthan 0.5% of the organic solvent.
 2. A process according to claim 1further comprising the step of spray drying the resulting solution toafford a stable amorphous solid micelle.
 3. A process according to claim1 further comprising the step of lyophilizing the resulting solution toafford a stable amorphous solid.
 4. A process according to claim 1wherein: said compound is a compound of formula I:

said organic solvent is tetrahydrofuran (THF); two equivalents of 1 MNaOH are added.
 5. A process according to claim 4 further comprising thestep of spray drying the aqueous micelle solution to afford a stableamorphous micellar solid.
 6. A process according to claim 1 wherein:said compound is a compound of formula II; and,

said solvent is iso-propanol (IPA)
 7. A process according to claim 6further comprising the step of spray drying the aqueous micelle solutionto afford a stable amorphous micellar solid.